A given standard Netsuite user login is limited to two sessions at the same time, one through the front-end (UI) and one through the web services.
For example, if someone attempts to log in from two different browsers at a time by using the same login user, the login from the second browser will terminate the first login.
The same behaviour will happen same as above for the web services client when two or more web services clients attempt to establish concurrent sessions, the latest log in will eventually cause termination to other accesses.
However, the front-end (UI) session and web services session does not cause termination of each other.
There is one alternative which allows concurrent web services session. This is can be done by having the Web Services Concurrent License (Web Services Plus) which allows for one designated user to create up to five concurrent Web services sessions. Further details about this feature can be found here, Web Services Plus.